Hydraulic jacks are widely used due to its advantages including compact structure, stable operation, light weight, portability, etc. In other words, hydraulic jacks are simple hydraulic lifting utilities, and can be used on vehicles as driver's tools; in addition, they are equipped in vehicle maintenance departments, vehicle test departments, fire departments and even engineering construction units. As known by those skilled in the art, the common drawbacks of hydraulic jacks lies in limited lifting height and low lifting speed, leading to low working efficiency; therefore, a great of effort has been made in the industry to overcome the drawbacks, as is manifested by the technical solution of the oil jack lifting to top in idle stroke with one pump disclosed in Chinese Utility Model Patent Publication No. CN 2331648Y. It seems that the load safety of hydraulic jacks has been neglected. The actual situation is: in some special cases, it is difficult to judge whether a lifting jack can jack up a heavy object because the load is not clear; especially in the case where the safety load limit unit of lifting jack is adjusted and thereby the protection of lifting jack is lost, overloading will damage the lifting jack, and even cause adverse chain reactions, for example, the lifted heavy object may drop suddenly and thereby be damaged, and even cause more severe accidents (e.g., personal safety is endangered).
A lifting jack with pressure gauge is provided in Chinese Utility Model Patent Publication No. CN2131847Y. On the lifting jack, a hole that communicates with the bottom of an oil chamber is arranged on the bottom of the base, one end of an oil tube is connected to the hole, and the other end of the oil tube protrudes from the base and is connected with a connector, on which a pressure gauge is mounted. A hydraulic jack capable of weighing and measuring force is introduced in Chinese Utility Model Patent Publication No. CN2371548Y. In the technical solution of CN2371548Y, the oil return valve on the lifting jack is removed and replaced with a pipe joint, wherein, one end of the pipe joint is fitted with the pipe of the oil circuit through thread connection, and the other end of the pipe joint is fitted with a pressure gauge through thread connection. A multi-purpose vertical hydraulic jack with a pressure gauge is put forward in CN2714529Y. The technical solution of the multi-purpose vertical hydraulic jack is: a pressure gauge interface is arranged at a side where the casing of lifting jack base and piston barrel interface is located; the pressure gauge interface communicates with the piston barrel through the oil circuit, and the pressure gauge is connected with the pressure gauge interface via a flexible pipe joint assembly (see line 10 on page 2 to line 2 on page 3 in the description).
A common advantage of the three above mentioned patents is that the load and pressure can be displayed when the lifting jack lifts up, therefore blind operation can be avoided, and the trouble can be nipped in the bud to a certain degree. However, there are undeniable common drawbacks: firstly, since “operations” have to be made to the base of lifting jack, the machining work is complicated; secondly, the leak tightness is a crucial factor for judging the quality of lifting jack and leakage (oil leakage) is taboo for lifting jacks, but in all three of the above-mentioned patents, the pressure gauge communicates with the oil circuit, and consequently increases the probability of leakage from lifting jack. It is inadvisable for the setting mode of pressure gauge to arrange the pressure gauge to communicate with the oil circuit in terms of technical feasibility and actual applicability. Not only does this apply to the leak tightness of pressure gauge positioning units, such as the connector, pipe joint, pressure gauge interface, and base joint as described in the three above-mentioned patents, but also the leak tightness between pressure gauge and connector, pipe joint, and pressure gauge interface, should be considered. In case leakage occurs in any of the sealing links, the lifting jack could not be used. Thirdly, the indication error of the pressure gauge is big, especially, when the observer observes at a position that is offset from the front of the pressure meter, the load judgment may be inaccurate due to viewing angle factor.